1. Field of the Invention
The present invention relates to a wavelength-selective element and specifically to a wavelength-selective element for switching the active optical path based on wavelength differences.
2. Description of the Background Art
Optical communication systems have various types. For example, a passive optical network (PON) communication system has been widely used, in which an optical line terminal (OLT) arranged in a communication carrier connects to a plurality of optical network units (ONUs) located on subscriber premises via optical fibers and star couplers so as to form an optical access system where the OLT is shared among the ONUs. In this system, a wavelength of optical signals used in communications for down-stream direction, i.e. down-stream communication, from the OLT to the ONUs is made different from a wavelength of optical signals used in communications for up-stream direction, i.e. up-stream communication, from the ONUs to the OLT in order to prevent interference of lightwaves from occurring between the down-stream and up-stream communications.
Each of OLT and ONUs is configured such that it includes optical elements such as a wavelength filter, a photodiode, a laser diode and so on. In order to spatially couple the optical components by using a lens, there needs a complex process for aligning respective optical axis, i.e. for aligning the center positions (light-receiving positions or light-emissive positions) of the optical elements. Accordingly, a simplified process is required which can lead to mass-producibility.
For such a simplified process, there is an available way to use an optical waveguide instead of the lens to couple spatially the optical components as disclosed by U.S. Pat. No. 4,860,294 B2 to Winzer, et al., U.S. Pat. No. 5,764,826 B2 to Kuhara, et al., U.S. Pat. No. 5,960,135 B2 to Ozawa, U.S. Pat. No. 7,072,541 B2 to Kim, et al., and Japanese patent laid-open publication No. 163028/1996, for example. They use the optical waveguide instead of the lens to couple the optical components and provide a path-switching element having mass-producibility.
The path-switching element has a function of switching between a path for optical signals in the down-stream communications and a path for optical signals in the up-stream communications. Specifically, the path-switching element selectively activates an optical path for optical signals used in the down-stream communications and an optical path for optical signals used in the up-stream communications so as to send the respective optical signals into a common single optical fiber. The path-switching element also has a function of a wavelength filter because it switches between the path for optical signals in the down-stream communications and the path for optical signals in the up-stream communications according to difference in wavelength.
In the case where the optical components are coupled by using the optical waveguide, light is allowed to propagate while confined within the optical waveguide whereby there is no necessity to do the above complex process which is required when the lens is used. By forming register marks for the optical components such as a wavelength filter, a photodiode, a laser diode and so on in a chip where the optical waveguide is formed, it is easy to place the respective centers of the optical elements to the optimum positions. Accordingly, by using the optical waveguide, it is possible to fabricate the path-switching elements having mass-producibility.
Such a path-switching element in the ONU and OLTs forming the PON communication system is tried to be constituted by a silicon wire optical waveguide which is made of silicon (Si) adapted for miniaturization and mass production as disclosed in Serge Bidnyk, et al., “Silicon-on-Insulator-Based Planar Circuit for Passive Optical Network Applications”, IEEE Photonics Technology Letters, vol. 18, No. 22, Nov. 15, 2006, pp. 2392-2394; Ning-Ning Feng, et al., “Low-Loss Polarization-Insensitive Silicon-on-Insulator-Based WDM Filter for Triplexer Applications”, IEEE Photonics Technology Letters, vol. 20, No. 23, Dec. 1, 2008, pp. 1968-1970; and Hsu-Hao Chang, et al., “Integrated hybrid silicon triplexer”, Optics Express, vol. 18, No. 23, Nov. 8, 2010, pp. 23891-23899, for example.
The silicon wire optical waveguide is also referred to as a silicon nanowire optical waveguide and has the structure where a silicon optical waveguide (core) is surrounded by a material (cladding layer) such as silicon oxide (SiO2) having its refractive index smaller than silicon. The refractive index of the silicon oxide forming the cladding layer is so different from the refractive index of the silicon forming the core that it is possible to confine almost all of optical electric field components within the core and hence to reduce the cross-sectional dimension of the core to a quite small size, for example, in the order of submicrons. Furthermore, the silicon wire optical waveguide can be made by using common processes for fabricating semiconductor devices or chips and thus be easily mass-produced.
In the case where the optical path-switching element is constituted by the silicon wire optical waveguide, a diffraction grating is commonly used for implementing the above-mentioned function of the wavelength filter and there is a necessity to fabricate a diffraction grating having much shorter period than the wavelength of the optical carrier wave of optical signals whose path will be switched by the path-switching element including the diffraction grating. For fabricating such a diffraction grating, however, a sophisticated technique is required due to its microscopic structure.
Also, there is a phenomenon where light components having wavelengths shorter than the Bragg reflection wavelength of the diffraction grating are given off to the outside from an optical waveguide forming the diffraction grating. It is therefore necessary to prevent the light components from radiating to the outside in some way or other when the path-switching device having the diffraction grating is mounted in an ONU or OLTs.